Volker,
check, if it is your Aperture library that is causing the problem. The library may need repairing or contain corrupted videos or photos, that cannot be processed.
Switch Aperture to a new, empty library. Import a few image files for testing. Quit Aperture. Try syncing again.
If that works,delete the iPod Photo Cache from your main Aperture library and try again. See this support document: iTunes: Understanding the iPod Photo Cache folder
If removing this cache does not help, use the first aid tools on the Aperture library.
- Launch Aperture with options-command keys held down.
- Keep holding down the keys firmly, until you are seeing the First Aid Panel.
- Select "repair database".
Back up the Aperture library, before you try the other first aid options.
-- Léonie